The formation of a contact for a semiconductor device is an important process in fabricating the semiconductor device. For example, contact resistance significantly affects the performance characteristics of the semiconductor device. As a result, achieving a low contact resistance is typically desired. FIGS. 1A-B show a structure 2 and equivalent circuit 6, respectively, of a typical annealed metal (e.g., Ohmic) contact 4 according to the prior art. In particular, contact 4 comprises a contact to a heterostructure with a two-dimensional (2D) electron gas at the heterointerface. In this case, the equivalent circuit 6 consists of resistive components. Therefore, the resistance of contact 4 is frequency independent.
However, in many high-frequency semiconductor devices, the resistance of contact 4 is a dominant factor in limiting the performance of the device. To date, a low contact resistance can be obtained by using high-temperature annealing in the contact formation. Such annealing remains a challenging technology, especially for wide bandgap semiconductor materials. In particular, as the material bandgap increases, the required contact annealing temperature increases, yet the contact resistance also increases. Additionally, contact formation becomes even more challenging with novel emerging wide bandgap material systems and devices, such as material systems/devices based on aluminum nitride (AlN), diamond, and others.
For example, Au/Ge/Ni/Au contacts to gallium arsenide (GaAs) or indium GaAs (InGaAs) require an annealing temperature in the range of 350-425 degrees Celsius and produce unit-width contact resistances as low as 0.1Ω-mm or even lower. For gallium nitride (GaN), contact formation using Ti/Al contacts requires an annealing temperature in the range of 800-850 degrees Celsius with typical unit-width contact resistances in the range of 0.5-1.0Ω-mm. While advanced annealing techniques have been shown to yield lower unit-width contact resistances, the general trend in which high annealing temperatures lead to significant morphology degradation, defect generation, and contact edge roughness remains.
FIGS. 2A-B show another structure 3 and equivalent circuit 7, respectively, of an alternative metal contact 5 according to the prior art. In particular, metal contact 5 comprises a conventional, under annealed contact, in which there is a capacitive coupling between the metal and the underlying two-dimensional electron gas. In this case, the capacitive coupling has been shown to reduce the access resistance to the two-dimensional electron gas at high frequencies (e.g., radio frequencies).
However, access resistance between a device channel and a contact region, where electrons enter into a narrow channel from a thick contact region, remains a large contributor to the overall contact resistance. While under annealed contact 5 has a smaller impedance at higher frequencies, it does not reduce the access resistance between the device channel and contact region. Further, under annealed contact 5 does not provide any ability to achieve self-aligned or alignment-free fabrication.